JP5544934B2 - Method for producing positive electrode active material for lithium ion battery - Google Patents

Description

The present invention relates to a method for producing a positive electrode active material for a lithium ion battery . More specifically, the present invention relates to a positive electrode active material for a lithium ion battery capable of easily and efficiently generating LiFePO4 fine particles having a nanometer size . It relates to a manufacturing method .

In recent years, the reduction of environmental impact has been screamed, and the flow of zero emission and oil-free society has been addressed nationwide. In particular, secondary batteries such as electric cars and portable electronic devices are in the limelight and are positioned as one of the fields responsible for realizing a clean energy society.
As typical secondary batteries currently used, for example, lead storage batteries, alkaline storage batteries, lithium ion batteries, and the like are known. Among these secondary batteries, in particular, lithium ion batteries can be reduced in size, weight and capacity, and have excellent characteristics such as high output and high energy density. Research and development of materials used in next-generation lithium-ion batteries are also active.

LiCoO ₂ is generally used as a positive electrode active material for lithium ion batteries currently in practical use. However, Co is unevenly distributed on the earth and is a scarce resource, and considering that it is used in large quantities as a positive electrode active material, there is a problem that stable supply is difficult, and thus the manufacturing cost of the product becomes high. .
Therefore, as a positive electrode active material replacing LiCoO ₂ , spinel-based LiMn ₂ O ₄ , ternary LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , lithium ferrate (LiFeO ₂ ), lithium iron phosphate Research and development of positive electrode active materials such as (LiFePO ₄ ) has been actively promoted.
Among these, LiFePO ₄ having an olivine structure contains phosphorus as a constituent element and is strongly covalently bonded to oxygen, so that it does not release oxygen at a high temperature as compared with a positive electrode material such as LiCoO ₂ . It has attracted attention as a positive electrode material that has no risk of ignition due to oxidative decomposition of the electrolyte and is excellent in safety as well as in terms of resources and costs (for example, Patent Document 1, Non-Patent Document 1) (See Patent Document 1).

However, even in LiFePO ₄ having such advantages, there is a problem that the capacity is reduced in terms of characteristics. The cause of this capacity reduction is the slow diffusion of lithium ions inside the active material derived from the olivine structure peculiar to LiFePO ₄ and the electron. It is considered to have low conductivity.
Therefore, as a measure for improvement, a method is generally used in which the particle size is reduced to about nanometers in order to shorten the Li diffusion distance, and the surface of the particles is coated with a carbon film having a thickness of 1 to 2 nm in order to improve electronic conductivity. .

Conventionally, a solid phase method has been used as a method for synthesizing LiFePO ₄ , and in this solid phase method, a raw material of LiFePO ₄ is mixed at a stoichiometric ratio to produce a calcined precursor, The particle diameter is adjusted by firing in an inert atmosphere and mechanically pulverizing the obtained sintered body.
However, in this solid phase method, since the sintered body is mechanically pulverized, it is difficult to control the particle size, and the operation process is very difficult. In addition, since a coarse sintered body is mechanically pulverized, particles having large shape anisotropy are easily generated, and it is difficult to obtain LiFePO ₄ fine particles having a uniform particle diameter.
Therefore, as a method for producing uniform and minute particles made of LiFePO ₄ , a liquid phase synthesis method utilizing a hydrothermal reaction has attracted attention.

The advantage of a hydrothermal reaction is that a product with a much higher purity is obtained at a much lower temperature compared to a solid phase reaction. However, also in this hydrothermal reaction, the particle size is largely controlled by factors relating to reaction conditions such as reaction temperature and time. In addition, when controlled by these factors, there are many parts that depend on the performance of the manufacturing apparatus itself, and reproducibility is difficult.
Therefore, as a method for producing LiFePO ₄ fine particles by a hydrothermal reaction, a method of synthesizing organic acids and ions such as CH ₃ COO ⁻ , SO ₄ ²⁻ , Cl ^{− and the} like by simultaneously containing them in a solvent, and this hydrothermal reaction There has been proposed a method of obtaining single-phase LiFePO ₄ fine particles by adding excess Li at the time (for example, see Patent Document 2, Non-Patent Document 2, etc.). A method of obtaining LiFePO ₄ fine particles having a small particle diameter by mechanically pulverizing a reaction intermediate has also been proposed (Patent Document 3).

Japanese Patent No. 3484003 JP 2008-66019 A Special table 2007-511458 gazette

A. K. Padi et al., “Phospho-Olibin as Positive-Electrode Material for Rechargeable Lithium Batteries”, Journal of the Electrochemical Society, 1997, Vol. 144, No. 4, p. 1188 (AKPadhi et al., “ Phosph0-olivine as Positive-Electrode Material for Rechargeable Lithium Batteries ", J. Electrochem. Soc., 144, 4, 1188 (1997)) Keisuke Shiraishi, Yoon Holo, Kiyoshi Kanamura, “Synthesis of LiFePO4 Cathode Active Material for Rechargeable Lithium Battery by Hydrothermal Reaction”, Journal of the Ceramic Society of Japan, 2004, Vol. 112, No. 1305, No. 1 S58-S62 (Keisuke Shiraishi, Young Ho Rho and Kiyoshi Kanamura, "Synthesis of LiFePO4 cathode active material for Rechargeable Lithium Batteries by Hydrothermal Reaction", J. Ceram. Soc. Jpn., Suppl. 112, S58-S62 (2004)

By the way, in the method of producing LiFePO ₄ fine particles by a conventional hydrothermal reaction, it is possible to make particles finer by adjusting the reaction temperature at the time of LiFePO ₄ synthesis and the stoichiometric ratio of raw materials. If the particles are too fine, many amorphous layers (amorphous layers) are present on the surface of the particles. As a result, the Li diffusibility inside the particles deteriorates due to a decrease in the crystallinity of the particles, and the charge is reduced. There was a problem that the discharge characteristics deteriorated.
Further, the LiFePO ₄ fine particles are easily oxidized by oxygen present in the air, and thus the oxidized LiFePO ₄ fine particles may adversely affect the battery characteristics.

The present invention was made in view of the above circumstances, and an object thereof is to provide a method for producing a cathode active material for a lithium ion battery that can be the size of the particles to produce a homogeneous LiFePO ₄ particles .

As a result of diligent research to solve the above-mentioned problems, the present inventors have focused on the solvothermal method, which has recently attracted attention as a method for synthesizing nanoparticles when synthesizing LiFePO ₄ fine particles. with incorporated into the hydrothermal synthesis reaction of LiFePO _4, a mixture of a raw material of LiFePO ₄ particles, in an atmosphere of an inert gas or reducing gas, 120 ° C. or higher and 250 ° C. or more below the temperature and 0.1MPa and 2.0MPa If the reaction is carried out under the following gauge pressure and the content of the water-soluble organic solvent contained in the mixture is 5 mass% or more and 60 mass% or less of the total mass of the mixture, the raw material water and water solubility The solubility in organic solvents can be increased and the surface oxidation of the generated particles can be suppressed. As a result, the average primary particle size is 30 nm or less. And it is the size of the is moreover particles in the range of 100nm is to produce easily uniform LiFePO ₄ particles, further applies the LiFePO ₄ particles obtained by this method in the positive electrode active material of the lithium ion battery Thus, it was found that the charge / discharge characteristics of the lithium ion battery can be improved, and the present invention has been completed.

That is, the method for producing a positive electrode active material for a lithium ion battery according to the present invention includes Li ₃ PO ₄ , or a Li source and a phosphoric acid source, an Fe source, water, and a water-soluble organic solvent having a boiling point of 150 ° C. or higher. The mixture to be reacted in an atmosphere of an inert gas or a reducing gas under a temperature of 120 ° C. or more and 250 ° C. or less and a gauge pressure of 0.1 MPa or more and 2.0 MPa or less, and the average primary particle size is 30 nm. The method for producing a positive electrode active material for a lithium ion battery that produces LiFePO ₄ fine particles of 100 nm or less, wherein the content of the water-soluble organic solvent is 5% by mass or more and 60% by mass or less of the total mass of the mixture. It is characterized by being.

The water-soluble organic solvent is preferably one or more selected from the group consisting of polyhydric alcohols, amides, esters and ethers .
The inert gas is one selected from the group of nitrogen gas, argon gas, helium gas and carbon dioxide gas, the reducing gas is carbon monoxide gas, and the high pressure is 0.1 MPa or more and It is preferable that it is 2.0 MPa or less.

According to the method for producing a positive electrode active material for a lithium ion battery of the present invention, Li ₃ PO ₄ , or a Li source and a phosphoric acid source, an Fe source, water, and a water-soluble organic solvent having a boiling point of 150 ° C. or more are contained. The mixture to be reacted in an atmosphere of an inert gas or a reducing gas under a temperature of 120 ° C. or higher and 250 ° C. or lower and a gauge pressure of 0.1 MPa or higher and 2.0 MPa or lower, and a water-soluble organic solvent Since the content is 5% by mass or more and 60% by mass or less of the total mass of the mixture, it is possible to increase the solubility of the raw material in water or a water-soluble organic solvent and to suppress surface oxidation of the generated particles. As a result, it is possible to easily and efficiently produce LiFePO ₄ fine particles having an average primary particle diameter in the range of 30 nm or more and 100 nm or less and having a uniform particle size.

It is a field effect type | mold scanning electron microscope (FE-SEM) image which shows the positive electrode active material of Example 1 of this invention. 3 is a field effect scanning electron microscope (FE-SEM) image showing a positive electrode active material of Comparative Example 1.

The manufacturing method of the positive electrode active material for lithium ion batteries of this invention, the electrode for lithium ion batteries, and the form for implementing a lithium ion battery are demonstrated.
This embodiment is specifically described for better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

"Method for producing positive electrode active material for lithium ion battery"
The method for producing a positive electrode active material for a lithium ion battery according to this embodiment includes Li ₃ PO ₄ , or a Li source and a phosphoric acid source, an Fe source, water, and a water-soluble organic solvent having a boiling point of 150 ° C. or higher. In this method, the mixture is reacted in an atmosphere of an inert gas or a reducing gas at high temperature and high pressure to produce LiFePO ₄ fine particles having an average primary particle size of 30 nm or more and 100 nm or less.

Here, when Li ₃ PO ₄ , or a mixture containing a Li source and a phosphoric acid source, an Fe source, and water or a water-soluble organic solvent is simply reacted in an inert gas atmosphere at normal pressure , resulting LiFePO ₄ particles can cause coarsened, it is impossible to average primary particle size obtain LiFePO ₄ particles having a uniform size of and 100nm following range of 30 nm. Therefore, reacting the above mixture in an atmosphere of an inert gas or a reducing gas at a high temperature and a high pressure is an important parameter in controlling the particle size of the LiFePO ₄ fine particles.
Therefore, when the above mixture is reacted in an atmosphere of an inert gas or a reducing gas, if the reaction is performed at a high temperature and a high pressure, the solubility of the reactant increases, so that the nucleation of the generated particles The frequency increases, and a large number of LiFePO ₄ fine particles having an extremely small particle diameter are obtained.

When the LiFePO ₄ fine particles are synthesized by hydrothermal reaction, a method using a Li source such as a Li salt, a Fe source such as a Fe (II) salt, a phosphoric acid source such as a PO ₄ salt as a synthesis raw material, a Li source and phosphorus There are a method using Li ₃ PO ₄ reacted with an acid source, and a method using Fe ₃ (PO ₄ ) ₂ reacted with an Fe source and a phosphate source.
However, since Fe ₃ (PO ₄ ) ₂ is easily oxidized and difficult to handle, it is preferable to use Li ₃ PO ₄ and an Fe (II) salt as raw materials.
Note that the method using Li source, Fe source and phosphoric acid source is equivalent to the method using Li ₃ PO ₄ because Li ₃ PO ₄ is generated at the initial stage of the reaction. Therefore, a method is preferred in which Li ₃ PO ₄ is first synthesized, and then Li ₃ PO ₄ and Fe source are hydrothermally reacted to synthesize LiFePO ₄ fine particles.

Next, a method for producing the LiFePO ₄ fine particles will be described in detail.
1. Production of Lithium Phosphate (Li ₃ PO ₄ ) First, a Li source and a phosphoric acid source are introduced into water, and the Li source and the phosphoric acid source are reacted to form lithium phosphate (Li ₃ PO ₄ ) in the solution. is generated, then this solution was added an alkaline solution to precipitate lithium phosphate (Li 3 _{PO 4),} the then unreacted substances were removed by performing cleaning, lithium high purity phosphoric acid (Li ₃ PO ₄ ) Collect.

The Li source is preferably Li hydroxide or Li salt. For example, Li hydroxide is lithium hydroxide (LiOH), and Li salt is lithium carbonate (Li ₂ CO ₃ ). , Lithium inorganic acid salts such as lithium chloride (LiCl) and lithium nitrate (LiNO ₃ ), lithium organic acid salts such as lithium acetate (LiCH ₃ COO) and lithium oxalate ((COOLi) ₂ ), and hydrates thereof 1 type or 2 types or more selected from these groups are used suitably.

Examples of phosphoric acid sources include phosphoric acid such as orthophosphoric acid (H ₃ PO ₄ ) and metaphosphoric acid (HPO ₃ ), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), and diammonium hydrogen phosphate ((NH ₄ )). ₂ HPO ₄ ), ammonium phosphate ((NH ₄ ) ₃ PO ₄ ) and one or more selected from the group of these hydrates are preferably used. Among them, orthophosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate are preferable because of their relatively high purity and easy composition control.

2. Preparation of mixture of lithium phosphate (Li ₃ PO ₄ ) and Fe source The above-described high purity lithium phosphate (Li ₃ PO ₄ ) is charged into a solution containing water and a water-soluble organic solvent having a boiling point of 150 ° C. or higher. And stirring to obtain a lithium phosphate (Li ₃ PO ₄ ) slurry. Next, an Fe source is added to the lithium phosphate (Li ₃ PO ₄ ) slurry to obtain a mixture.
The Fe source is preferably an Fe salt, such as iron chloride (II) (FeCl ₂ ), iron sulfate (II) (FeSO ₄ ), iron acetate (II) (Fe (CH ₃ COO) ₂ ), and water thereof. One or more selected from the group of Japanese products are preferably used.

The reaction concentration, that is, the concentration when Li ₃ PO ₄ and the Fe source in this mixture are converted to LiFePO ₄ is preferably 0.5 mol / L or more and 2.0 mol / L or less.
The reason is that when the reaction concentration is less than 0.5 mol / L, LiFePO ₄ having a large particle size is likely to be produced, and the capacity may be reduced. On the other hand, when the reaction concentration exceeds 2.0 mol / L, Since the viscosity of the raw material becomes high, the stirring cannot be sufficiently performed, and therefore the reaction does not proceed sufficiently and unreacted substances remain, making it difficult to obtain single-phase LiFePO _4. This is because it cannot be used as a battery material.

As a solvent for this mixture, it is necessary to use a mixed solvent of water and a water-soluble organic solvent having a boiling point of 150 ° C. or higher.
Examples of the water-soluble organic solvent include polyhydric alcohols such as ethylene glycol, propylene glycol, hexylene glycol and glycerin, amides such as formamide, N-methylformamide, 2-pyrrolidone and N-methyl-2-pyrrolidinone, γ -Only 1 type selected from the group of esters, such as butyrolactone, ethers, such as diethylene glycol and ethylene glycol monobutyl ether (butyl cellosolve), or 2 or more types can be mixed and used.
Of these, polyhydric alcohols are preferable, and ethylene glycol is particularly preferable.

In this mixed solvent, a part of water is a water-soluble organic solvent, and the content of the water-soluble organic solvent is 5% by mass or more and 80% by mass or less, preferably 20% by mass of the total mass of the hydrothermal reaction. It can be obtained by substitution so as to be not less than 30% by mass and not more than 30% by mass.
Here, if the substitution amount is less than 5% by mass, the generated LiFePO ₄ fine particles and the battery characteristics are almost the same as when the solvent is only water, and the substitution effect cannot be obtained. If the amount exceeds 80% by mass, the content of water is too small, so that a salt of a by-product is precipitated during the synthesis, and this salt becomes an impurity and deteriorates battery characteristics.
Thus, the substitution amount of the water-soluble organic solvent is defined with respect to the total mass of the hydrothermal reaction charged, that is, the total mass of the mixture. For example, 5% by mass means that 5% by mass of the total charged mass of the hydrothermal reaction is replaced with water in the reaction system.

3. Hydrothermal synthesis of the mixture The above mixture is put into a pressure resistant reaction vessel, and an inert gas or a reducing gas is 0.1 MPa or more and 2.0 MPa or less, preferably 0.5 MPa or more and in the pressure resistant reaction vessel. Pressurized and sealed to a pressure of 1.0 MPa or less, reacted (hydrothermal synthesis) in an atmosphere of this inert gas or reducing gas under high temperature and high pressure, and an average primary particle size of 30 nm or more and LiFePO ₄ fine particles of 100 nm or less are generated.

The inert gas is not particularly limited as long as it can prevent the oxidation of the surface of LiFePO ₄ fine particles having a very small particle diameter generated during the reaction (hydrothermal synthesis). One selected from the group of gas, helium gas, and carbon dioxide gas (carbon dioxide gas) is preferably used.
As the reducing gas, any one of carbon monoxide gas and hydrogen gas is preferably used. This reducing gas can also be used as a mixed gas mixed with an inert gas. Examples of the mixed gas include mixed gases such as 95 v / v% nitrogen gas-5 v / v% carbon monoxide gas.

The high temperature and high pressure conditions are not particularly limited as long as the temperature, pressure, and time are within the range of producing LiFePO ₄ fine particles with uniform particle sizes, but the reaction temperature is the water-soluble organic solvent to be substituted. For example, the temperature is preferably 120 ° C. or higher and 250 ° C. or lower, more preferably 150 ° C. or higher and 220 ° C. or lower.
Here, the reason why the reaction temperature does not exceed the boiling point of the water-soluble organic solvent to be substituted is that the water-soluble organic solvent decomposes and reacts when exposed to high temperature conditions that greatly exceed the boiling point in the pressure vessel. This is because the internal pressure in the container is suddenly increased, which may cause a safety problem.

Moreover, the pressure at the time of reaction can be known by the gauge pressure in a pressure-resistant reaction container, 1.5 MPa or more and 3.5 MPa or less are preferable, More preferably, it is 2.0 MPa or more and 3.0 MPa or less.
In particular, when an inert gas or a reducing gas is pressurized and sealed in a pressure-resistant reaction vessel, the pressure resistance reaction takes into account the vapor pressure of the mixture at high temperature and the pressure pressurized by the inert gas or reducing gas. It is necessary to adjust the pressurization of the inert gas or the reducing gas so as not to exceed the pressure resistance in the container.
Although depending on the reaction temperature, the reaction time is preferably, for example, 1 hour to 24 hours, more preferably 3 hours to 12 hours.

4). The reactions containing LiFePO ₄ particles separated above LiFePO ₄ particles, decantation, centrifugation, by filtration or the like, is separated into LiFePO ₄ particles and Li-containing waste solution (solution containing Li unreacted).
The separated LiFePO ₄ fine particles are dried at 40 ° C. or more for 3 hours or more using a drier or the like to obtain LiFePO ₄ fine particles having an average primary particle diameter of 30 nm or more and 100 nm or less.
As described above, LiFePO ₄ fine particles having a small average primary particle size and a narrow particle size distribution can be obtained efficiently.

The LiFePO ₄ fine particles obtained in this way are used as a positive electrode active material for a lithium ion battery, so that the Li diffusion distance is shortened, and the lithium ion battery electrode and lithium provided with this positive electrode active material for a lithium ion battery In the ion battery, high-speed charge / discharge characteristics can be improved.

Here, when the average primary particle diameter of the LiFePO ₄ fine particles is less than 30 nm, the crystallinity is lowered, so that the effect of insertion / desorption of Li is lowered, and as a result, the charge / discharge capacity is lowered. On the other hand, if the average primary particle diameter exceeds 100 nm, the internal resistance of the positive electrode active material is increased, the movement of Li ions is also delayed, and as a result, problems such as a decrease in discharge capacity may occur. It is not preferable.

"Electrode for lithium ion battery and lithium ion battery"
Since the above-mentioned LiFePO ₄ fine particles have low electric conductivity, the surface of LiFePO ₄ fine particles is coated with carbon to be used as a positive electrode active material of a positive electrode of a lithium ion battery, particularly a lithium ion secondary battery. Need to be increased.
By covering the surface of the LiFePO ₄ fine particles with carbon, the electrical conductivity can be increased, and the charge / discharge characteristics can be remarkably improved.

As a carbon coating method, for example, LiFePO ₄ fine particles are mixed with a water-soluble monosaccharide, polysaccharide, or water-soluble polymer compound that is a carbon source, followed by evaporation to dryness, vacuum drying, or spray drying. Using a drying method such as freeze drying, a film is uniformly formed on the surface of the LiFePO ₄ fine particles, and then baked in an inert atmosphere at a temperature at which the carbon source decomposes to generate carbon. During this firing process, the carbon source is decomposed, and a conductive carbon film is formed on the surface of the LiFePO ₄ fine particles. Therefore, carbon-coated LiFePO ₄ fine particles can be generated.

The firing temperature depends on the type of carbon source, but is preferably 500 ° C to 1000 ° C, more preferably 700 ° C to 800 ° C.
At a low temperature of less than 500 ° C., the carbon source is not sufficiently decomposed and a conductive carbon film is not sufficiently formed, which becomes a resistance factor in the battery and adversely affects battery characteristics. On the other hand, at a high temperature exceeding 1000 ° C., the growth of LiFePO ₄ fine particles progresses and becomes coarse, and the high-speed charge / discharge characteristics due to the Li diffusion rate, which is a problem of LiFePO ₄ particles, are remarkably deteriorated.

Thus, by covering the above LiFePO ₄ fine particles with carbon, the positive electrode active material of a positive electrode of a lithium ion battery, particularly a lithium ion secondary battery, which has a high electrical conductivity and dramatically improved charge / discharge characteristics. Can be used.
A lithium ion battery can be obtained by using the electrode formed using the carbon-coated LiFePO ₄ fine particles as a positive electrode, and further including a negative electrode, an electrolyte, and a separator.

In this lithium ion battery, the positive electrode is formed using carbon-coated LiFePO ₄ fine particles in which the surface of LiFePO ₄ fine particles having an average primary particle diameter of 30 nm or more and 100 nm or less is coated with a conductive carbon film. Therefore, the charge / discharge characteristics are improved, and the battery characteristics at a high rate are also improved.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention concretely, this invention is not limited by these Examples.

"Example 1"
To 1 L of pure water, 3 mol of lithium chloride (LiCl) and 1 mol of phosphoric acid (H ₃ PO ₄ ) were added and stirred to obtain a lithium phosphate (Li ₃ PO ₄ ) slurry.
Next, 1 mol of iron (II) sulfate (FeSO ₄ ) is added to the slurry, and a part of the water in the slurry is replaced with ethylene glycol so that the mass becomes 20% by mass with respect to the total charged mass of the hydrothermal reaction. The reaction concentration of this slurry was 1.5 mol / L in terms of LiFePO ₄ .

Next, this slurry was put into a pressure resistant reactor, nitrogen gas was used as an inert gas, and this nitrogen gas was pressurized and sealed in the pressure resistant reactor. The reaction was started by adjusting the pressure in the pressure-resistant reaction vessel at the time of sealing so that the gauge pressure was 0.5 MPa.
Next, the temperature in the pressure-resistant reaction vessel was raised to 180 ° C. over 2 hours, and heated at 180 ° C. for 6 hours to generate LiFePO ₄ fine particles. The gauge pressure during this constant temperature heating was 1.5 MPa. Then, it filtered and separated into solid and liquid.

The obtained cake-like LiFePO ₄ is pulverized and dispersed for 12 hours in a ball mill using 5 mm diameter zirconia beads as a medium after adding 5 g of polyethylene glycol and 150 g of pure water to 150 g in terms of solid content. A slurry was prepared.
Next, the slurry was sprayed into an air atmosphere at 180 ° C. and dried to obtain a granulated body having an average particle size of about 6 μm. This granulated body was fired at 750 ° C. for 1 hour under an inert atmosphere to prepare a positive electrode active material for a lithium ion battery of Example 1.

"Example 2"
The positive electrode active material for a lithium ion battery of Example 2 according to Example 1 except that the reaction was started by adjusting the pressure in the pressure resistant reactor at the time of nitrogen gas filling so that the gauge pressure was 1.0 MPa. Was made.

"Example 3"
The positive electrode active material for a lithium ion battery of Example 3 according to Example 1 except that the reaction was started by adjusting the pressure in the pressure resistant reactor at the time of nitrogen gas filling so that the gauge pressure was 2.0 MPa. Was made.

Example 4
A part of the water in the lithium phosphate (Li ₃ PO ₄ ) slurry is replaced with ethylene glycol so that the amount becomes 10% by mass with respect to the total charged mass of the hydrothermal reaction. A positive electrode active material for a lithium ion battery of Example 4 was prepared according to Example 1 except that the reaction was started by adjusting the pressure so that the gauge pressure was 2.0 MPa.

"Example 5"
A part of the water in the lithium phosphate (Li ₃ PO ₄ ) slurry is replaced with ethylene glycol so that the amount becomes 40% by mass with respect to the total charged mass of the hydrothermal reaction. A positive electrode active material for a lithium ion battery of Example 5 was prepared according to Example 1 except that the reaction was started by adjusting the pressure so that the gauge pressure was 2.0 MPa.

"Example 6"
A part of the water in the lithium phosphate (Li ₃ PO ₄ ) slurry is replaced with ethylene glycol so that the amount becomes 60% by mass with respect to the total charged mass of the hydrothermal reaction. A positive electrode active material for a lithium ion battery of Example 6 was produced according to Example 1 except that the reaction was started by adjusting the pressure so that the gauge pressure was 2.0 MPa.

" Reference Example 1 "
A part of the water in the lithium phosphate (Li ₃ PO ₄ ) slurry was replaced with ethylene glycol so that the amount was 80% by mass with respect to the total charged mass of the hydrothermal reaction, A positive electrode active material for a lithium ion battery of Reference Example 1 was prepared according to Example 1 except that the reaction was started by adjusting the pressure so that the gauge pressure was 2.0 MPa.

" Example 7 "
Argon gas is used as inert gas, except that the gauge pressure the pressure in the pressure-resistant reaction vessel when the argon gas filled has to initiate the reaction adjusted to 2.0MPa is carried out in accordance with Example 1 The positive electrode active material for lithium ion batteries of Example 7 was produced.

" Example 8 "
Helium gas used as the inert gas, except that the gauge pressure of the pressure in the pressure-resistant reaction vessel when the helium gas sealing is to initiate the reaction adjusted to 2.0MPa is carried out in accordance with Example 1 The positive electrode active material for lithium ion batteries of Example 8 was produced.

" Example 9 "
Example 1 except that carbon monoxide gas was used as the inert gas, and the reaction was started by adjusting the pressure in the pressure-resistant reaction vessel at the time of filling the carbon monoxide gas so that the gauge pressure was 2.0 MPa. The positive electrode active material for a lithium ion battery of Example 9 was produced according to the above.

" Example 10 "
According to Example 1, except that carbon dioxide gas was used as the inert gas, and the reaction was started by adjusting the pressure in the pressure-resistant reaction vessel at the time of carbon dioxide gas filling so that the gauge pressure was 2.0 MPa. Thus, a positive electrode active material for a lithium ion battery of Example 10 was produced.

" Example 11 "
A part of the water in the lithium phosphate (Li ₃ PO ₄ ) slurry is replaced with glycerin so that the amount is 20% by mass with respect to the total mass of the hydrothermal reaction, and the reaction concentration of this slurry is converted to LiFePO ₄ . Example 11 was carried out in accordance with Example 1 except that the reaction was started by adjusting the pressure in the pressure-resistant reaction vessel at the time of nitrogen gas filling to 1.5 mol / L so that the gauge pressure was 2.0 MPa. A positive electrode active material for a lithium ion battery was prepared.

" Example 12 "
A part of the water in the lithium phosphate (Li ₃ PO ₄ ) slurry was replaced with formamide so that it would be 20% by mass with respect to the total hydrothermal reaction charge mass, and the reaction concentration of this slurry was converted to LiFePO ₄ Except that the reaction was started by adjusting the pressure in the pressure-resistant reaction vessel at the time of nitrogen gas filling to 1.5 mol / L so that the gauge pressure was 2.0 MPa, the reaction of Example 12 was performed according to Example 1. A positive electrode active material for a lithium ion battery was prepared.

" Example 13 "
A part of the water in the lithium phosphate (Li ₃ PO ₄ ) slurry was replaced with γ-butyrolactone so as to be 20 mass% with respect to the total hydrothermal reaction charge mass, and the reaction concentration of this slurry was changed to LiFePO _4. and 1.5 mol / L in terms, except that the pressure in the pressure-resistant reaction vessel during nitrogen gas sealed gauge pressure was initiated adjusted to react so as to 2.0 MPa, carried out in accordance with example 1 example Thirteen positive electrode active materials for lithium ion batteries were produced.

"Comparative Example 1"
A part of the water in the lithium phosphate (Li ₃ PO ₄ ) slurry is replaced with ethylene glycol so that the amount becomes 20% by mass with respect to the total charged mass of the hydrothermal reaction. A positive electrode active material for a lithium ion battery of Comparative Example 1 was produced according to Example 1 except that the reaction was started by adjusting the pressure so that the gauge pressure was 0.0 MPa (= atmospheric pressure).

"Comparative Example 2"
The lithium phosphate (Li ₃ PO ₄ ) slurry was put into a pressure-resistant reaction vessel as it was without replacing part of the water with ethylene glycol, and air was used instead of inert gas. The positive electrode active material for the lithium ion battery of Comparative Example 2 according to Example 1 except that the reaction was started by adjusting the pressure in the pressure resistant reactor so that the gauge pressure was 0.0 MPa (= atmospheric pressure). Was made.

“Comparative Example 3”
Lithium phosphate (Li ₃ PO ₄ ) slurry is put into a pressure-resistant reaction vessel as it is without replacing a part of the water with ethylene glycol, and the atmosphere is pressurized instead of the inert gas. The positive electrode active material for the lithium ion battery of Comparative Example 3 according to Example 1 except that the reaction was started by adjusting the pressure in the pressure resistant reaction vessel when sealed so that the gauge pressure was 0.5 MPa. Was made.

“Comparative Example 4”
Lithium phosphate (Li ₃ PO ₄ ) slurry is put into a pressure-resistant reaction vessel as it is without replacing a part of the water with ethylene glycol, and the atmosphere is pressurized instead of the inert gas. The positive electrode active material for the lithium ion battery of Comparative Example 4 according to Example 1 except that the reaction was started by adjusting the pressure in the pressure resistant reaction vessel when sealed so that the gauge pressure was 2.0 MPa. Was made.

"Evaluation of positive electrode active materials for lithium-ion batteries"
About each positive electrode active material of Examples 1-13, Reference example 1, and Comparative Examples 1-4, the average primary particle diameter and specific surface area were measured with the following method.
(1) Average primary particle diameter A 50,000-fold field effect scanning electron microscope (FE-SEM) image was taken with a field effect scanning electron microscope (FE-SEM), and from one field of view of this FE-SEM image. Twenty fine particles were randomly selected, and the average value of the measured particle sizes of these fine particles was defined as the average primary particle size.

(2) Specific surface area Specific surface area meter The specific surface area (m2 / g) of the positive electrode active material was measured using Belsorb II (manufactured by Nippon Bell Co., Ltd.).
Table 1 shows the characteristics of the positive electrode active materials of Examples 1 to 13, Reference Example 1 and Comparative Examples 1 to 4.
Moreover, the field effect scanning electron microscope (FE-SEM) image of the positive electrode active material of Example 1 is shown in FIG. 1, and the field effect scanning electron microscope (FE-SEM) image of the positive electrode active material of Comparative Example 1 is shown in FIG. Each is shown in FIG.

"Production of lithium ion secondary battery"
About the positive electrode active material of each of Examples 1 to 13, Reference Example 1 and Comparative Examples 1 to 4, the following treatment was performed, and each of the lithium ion secondary batteries of Examples 1 to 13, Reference Example 1 and Comparative Examples 1 to 4 was performed. Was made.
First, 90 parts by mass of the positive electrode active material, 5 parts by mass of acetylene black as a conductive auxiliary agent, 5 parts by mass of polyvinylidene fluoride (PVDF) as a binder, and N-methyl-2-pyrrolidinone (NMP) as a solvent were mixed. .
Next, these were kneaded using a three-roll mill to produce a positive electrode active material paste.

Next, this positive electrode active material paste was applied onto an aluminum current collector foil having a thickness of 30 μm, and then dried under reduced pressure at 100 ° C. to produce a positive electrode having a thickness of 30 μm.
Next, this positive electrode was punched into a 2 cm ² disk shape, dried under reduced pressure, and then a lithium ion secondary battery was produced using a stainless steel 2016 type coin cell in a dry argon atmosphere.
Here, metallic lithium was mixed in the negative electrode, a porous polypropylene film was mixed in the separator, and 1 mol of LiPF ₆ was mixed in the electrolyte with a solution of ethylene carbonate (EC) and diethyl carbonate (DEC) mixed at 3: 7. The mixture was used.

"Battery charge / discharge test"
A battery charge / discharge test was performed using each of the lithium ion secondary batteries of Examples 1 to 13, Reference Example 1 and Comparative Examples 1 to 4.
Here, the cutoff voltage was 2.0 V to 4.0 V, and the initial discharge capacity was measured by charging at 0.1 C and discharging at 0.1 C. The other discharge capacities were measured by charging at 0.2C and measuring the discharge capacities at 0.2C, 5C, 8C, and 12C.
The ratio (%) between the discharge capacity at 5C and the discharge capacity at 0.2C was defined as the discharge maintenance ratio (5C / 0.2C maintenance ratio).
Table 2 shows the discharge capacities and discharge maintenance rates (5C / 0.2C maintenance rates) of Examples 1 to 13, Reference Example 1 and Comparative Examples 1 to 4.

According to Tables 1 and 2 and FIGS.
(1) A portion of the water in the slurry is replaced with a water-soluble organic solvent, and the resulting slurry is put into a pressure-resistant reaction vessel, and an inert gas or a reducing gas is added to the pressure-resistant reaction vessel at 0.1 MPa. The inert gas having a higher pressure than the case of high temperature and no pressure encapsulation is performed by pressurizing and encapsulating at a pressure of 2.0 MPa or less and then reacting (hydrothermal synthesis) at high temperature and high pressure. It was confirmed that the reaction (hydrothermal synthesis) can be performed in a reducing gas atmosphere, and the average primary particle diameter of the positive electrode active material can be controlled in the range of 30 nm to 100 nm.

On the other hand, when an inert gas is filled in a pressure-resistant reaction vessel and synthesis is started in a state where the pressure of the inert gas is equal to atmospheric pressure, coarse particles are generated, and the average primary particle size is 100 nm or less. It was found that no microparticles could be obtained.
Therefore, pressurize and seal the inert gas or reducing gas so that it is at least under a pressure higher than the atmosphere, maintain the pressure in the pressure-resistant reaction vessel at a high level, and start synthesis, the average primary particle size It was confirmed that a positive electrode active material having a thickness of 30 nm to 100 nm can be easily generated.

(2) In the positive electrode active materials of Examples 1 to 3, the content of the water-soluble organic solvent in the lithium phosphate (Li ₃ PO ₄ ) slurry was 20% by mass, and the synthesis was performed under high pressure. The specific surface area is increased as compared with the positive electrode active material of Example 1, and the discharge capacity and the discharge maintenance ratio (5C / 0.2C maintenance ratio) are improved as compared with the lithium ion secondary battery of Comparative Example 1. Thereby, it was confirmed that the charge / discharge characteristics were improved and the initial discharge capacity was ensured.

The method for producing a positive electrode active material for a lithium ion battery according to the present invention includes Li ₃ PO ₄ , or a mixture containing a Li source and a phosphoric acid source, an Fe source, water, and a water-soluble organic solvent having a boiling point of 150 ° C. or higher. In the atmosphere of an inert gas or a reducing gas at high temperature and high pressure to produce LiFePO ₄ fine particles having an average primary particle size of 30 nm or more and 100 nm or less. By applying the positive electrode active material for a lithium ion battery, in particular, to the positive electrode of a lithium ion secondary battery, it is possible to improve the charge / discharge characteristics and the characteristics at a high rate. As a result, load characteristics are required. It can be expected that a lithium ion secondary battery that can be mounted on a power tool or a hybrid vehicle will be put to practical use, and its industrial significance is extremely large.

Claims (3)

Li ₃ PO ₄ , or a mixture containing a Li source and a phosphoric acid source, an Fe source, water, and a water-soluble organic solvent having a boiling point of 150 ° C. or higher, in an inert gas or reducing gas atmosphere at 120 ° C. The positive electrode active for a lithium ion battery which produces LiFePO ₄ fine particles having an average primary particle diameter of 30 nm or more and 100 nm or less by reacting at a temperature of 250 ° C. or less and a gauge pressure of 0.1 MPa or more and 2.0 MPa or less. A method for producing a substance , comprising:
Content of the said water-soluble organic solvent is 5 mass% or more and 60 mass% or less of the said mixture total mass, The manufacturing method of the positive electrode active material for lithium ion batteries characterized by the above-mentioned.   2. The positive electrode active for lithium ion battery according to claim 1, wherein the water-soluble organic solvent is one or more selected from the group consisting of polyhydric alcohols, amides, esters and ethers. A method for producing a substance.   3. The inert gas is one selected from the group consisting of nitrogen gas, argon gas, helium gas and carbon dioxide gas, and the reducing gas is carbon monoxide gas. The manufacturing method of the positive electrode active material for lithium batteries of description.